Theories of the EarWs Physical Evolution. 373 



In this hydrothermal action then, Captain Dutton finds a probable 

 cause for changes in the density of the nether rocks. A decrease of 

 density, accompanied with an expansion of volume, should occur 

 when water finds access to them at high temperatures ; an increase 

 of density with diminished volume should take place, when from 

 any cause new and stable crystalline compounds are formed. In 

 truth any chemical change below the surface of the earth should be 

 accompanied, as all reactions are, by changes of volume in the 

 reagents. Between the amorphous and crystalline states of common 

 minerals, the change would average one-tenth, or one-eighth, and 

 would be materially increased by the addition or loss of water — a 

 change far exceeding that which would result from any probable 

 change of temperature. 



Masses of crystalline rocks, which have been subjected to hydro- 

 thermal action, show, by the convolution of their layers, that they 

 have been evidently rendered plastic by such action; while 

 lavas are quite liquid. Daubree believed that this plastic condition 

 might exist at as low a temperature as 600^ F. The same con- 

 clusion may be reached by the consideration, that sedimentary rocks, 

 while undergoing metamorphism, have been evidently rendered 

 plastic, and it is impossible that they can have been buried deep 

 enough to have been so rendered by heat in the dry way. 



The author's theory consequently rests upon the basis of the 

 following propositions. 1. The hydrothermal theory of meta- 

 morphism is taken for granted. 2. That process consists in an 

 intensified solvent power of water over rock materials under great 

 pressure, and at a temperature approaching to redness. 3. This 

 state of silica, alumina, etc., is presumed to be essentially the same 

 as when those oxides are obtained in the laboratory in the soluble 

 hydrous condition, in which their specific gravity is considerably less 

 than in the crystalline anhydrous form. 4. Hence it is inferred, that 

 the condition of hydrothermal solution is attended with a large 

 diminution of specific gravity. 5. Inversely, a fall of tempera- 

 ture, or decrease of pressure, is followed by the crystallization of the 

 materials, and an increase of density. 



If the deeply buried rocks are composed, like those at the surface, 

 of silica, alumina, and the alkaline and earthy bases, it would require 

 but a very moderate degree of expansion to render their specific 

 gravity less than that of the rocks above them. If, when in this 

 expanded condition, they become also plastic, then the strata resting 

 on them will be in unstable equilibrium. 



So long as the foundations of the [upper] strata are rigid, or 

 denser than them, expansion would be attended by no further dis- 

 turbance than a vertical upward movement, and such minor com- 

 plications as might arise from inequality of the process, or of dis- 

 tribution of the load. But as soon as the density of the underlying 

 layer becomes less, a new order of movements must ensue. It has 

 already been remarked that the coherence of a mass of strata of great 

 extent is practically nothing. In masses of less extent, as in a single 

 mountain ridge, it would be appreciable. Hence under the conditions 



